The invention relates to a microrheoscopic detector for gases, especially for infrared gas analysis apparatus, comprising a heater wire and two resistance thermometers, which are disposed in the area of influence of the heater wire, and which consist of thin wire and are held between two thicker connecting wires.
In a number of physical measuring processes, it is necessary to measure very small pulsating pressures or flows of very low volume in gases. Such processes include, for example, infrared gas analysis, leakage measurements in low sensitivity ranges, breathing measurements in medicine, and other measurements involving processes in which very small amounts of gas are consumed.
For the measurement of the rate of flow of gases, microrheoscopic detectors of the kind initially described can be used which are also known as hot wire anemometers. High speed microrheoscopic detectors for gases are defined as a means of measuring the flow of very small volumes, the functioning elements of which are constituted by two or more, partially low-mass solid structures which are temperature-coupled through a gaseous path; by means of a heated solid body, which can be a relatively inert, massive part, a hot cloud is formed in the gas to be measured, and can be described by a pattern of isotherms. A forced flow deforms this gas cloud or the isotherm pattern. By means of one or more low-mass temperature sensors disposed within the gas cloud, a signal can be produced which is proportional, within limits, to the flow.
The requirement of low mass inertia in the temperature sensor results, for example, in resistance thermometers having extremely thin resistance wires whose diameter is between about 0.5 and 5.times.10.sup.-3 mm. A low mass inertia is necessary in order that the resistance thermometers may have a sufficient resolving power in the order of magnitude between about 10 and 50 Hz.
In a rheoscopic detector for very small flows, care must also be taken to see that no free convection can take place within the sensor, which would be contrary to the forced convection of the measuring effect. This requires that the size of the active sensor volume be on the order of approximately 1 mm.sup.3 and less. The length of the resistance thermometer in this case is to be between 0.4 and 1.5 mm. From the above-stated requirements and design standards it is apparent that a microrheoscopic detector of this kind is a product of delicate manufacturing technology involving stringent precision requirements. This necessarily entails high manufacturing expense involving considerable labor costs.
A microrheoscopic detector of the kind described in the beginning has become known through the dissertation presented by Gunter Schunck entitled, "Schnelle Messfuhler fur kleine Gasstrome," on Dec. 9, 1974, at the Electrical Engineering faculty of the University of Karlsruhe. In the known microrheoscopic detector, the heating conductors and the two resistance thermometers are suspended independently of one another on wire leads which permit thermal expansion on the basis of their resilient construction. While a microrheoscopic detector of this kind completely satisfies the technical requirements, experience has shown that it is sensitive to vibration and is expensive to manufacture. The vibrations which are inevitable in many measuring instruments, at times result in the destruction of the delicate resistance thermometers. Problems are also encountered in assembly, since the resistance thermometers have to be fastened individually to fusion-embedded wire leads, without placing any mechanical strain on them.